On average, it took 35 days for the overcharged customers to receive a refund, but more than a third of those out of pocket had to wait for over two months, and seven per cent are either still waiting or have given up on getting their money back.

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HOW THIS IS MONEY CAN HELP

The cost of billing errors may be even higher than the total of £102million, since more than a quarter of consumers admit they don't read their energy bills properly.

Some say they never check fees.

Highlighting the problem, sister title Money Mail was inundated with complaints from former Extra Energy customers, after revealing the supplier was using debt collectors to chase households for money they didn't owe.

It has been under investigation by the energy watchdog for more than a year over bill blunders and complaints handling.

Enraged customers included Rebecca Baker, 43, a compliance officer from Stoke-on-Trent, left Extra Energy last November and was sent a final bill for £149, when she owed only £78.

About one in three of customers affected said they were charged for an amount that did not match their meter readings, while a quarter said their energy bills did not add up correctly

The firm agreed to correct this, but in July, she started getting chased by a debt agency, which claimed she owed £932.

Others also aired their grievances over misbilling.

Claire Osborne, energy expert at uSwitch, said: 'Customers having to pay the price for suppliers' mistakes is unacceptable.

'Households are already feeling the pinch from recent energy price rises, and having to chase for an average of 35 days to get their money back simply adds insult to injury.'

HOW TO GET YOUR MONEY BACK FROM YOUR ENERGY PROVIDER

If you realise you are in credit to your energy supplier, you have the right to ask for this money back.

Often this will happen during the summer months if you pay by direct debit as you will be using less energy.

Energy companies will often tell you that as your bills will be higher in the winter it's worth keeping the money in the account, but if you want to use if for something else, and you know you will still be able to meet the higher bills in the colder months, you need to call up and ask for a refund.

If it refuses, you can make an official complaint and then escalate the complaint to the Ombudsman if necessary.

The same applies if you've switched suppliers and you were in credit at the time. Your old supplier should automatically refund you the money but it's always worth checking as sometimes this isn't the case.

The findings come just weeks after British Gas became the latest Big Six energy supplier to hike prices.

It is ramping up the cost of electricity by 12.5 per cent for 3.1million customers, despite falling wholesale prices.

Uswitch is calling for market regulator Ofgem and suppliers to dramatically reduce billing errors, reimburse customers within a month and issue bills that are easier to understand - one in five who do not read their bills say it is because they are too complicated.

Ms Osborne added: 'We want to see companies do much more to make life easier for their customers.

'Accurate bills are the bare minimum they should expect from their energy suppliers.'

Suggested improvements to help consumers spot mistakes include explanations on how the bill has been calculated, whether it was based on an actual meter reading, if the account is in credit and reminders customers could save by switching tariff or supplier.

An Ofgem spokesman said: 'We have taken strong action to address billing failures by suppliers, with suppliers paying out over £40million as a result of our investigations into these and related customer service issues in the last two years, and we continue to monitor the market.

'Suppliers are required by Ofgem to treat their customers fairly - if a customer thinks that their supplier has made a billing mistake, they should contact them.

'We are working with suppliers to make bills easier to understand and Ofgem's rules already require suppliers to make clear on bills whether they are based on a customer's actual or estimated energy usage.

'Smart meters will address the issue of estimated readings as these consumers will be billed based on their actual energy usage.'